For instance, there are grammar/spelling nazi trolls. My response is something along the lines of, "The guy made a common mistake, this forum is not a peer-reviewed journal where you have to be particularly careful, and the rest of us were smart figure out exactly what he meant. Are you too stupid to work it out for yourself?"

If the person can't help making grammar mistakes it is alright. But often times the writer could write perfectly well but doesn't care to put in the effort. This is not some throw away chat session where are few mistakes are okay. This will be read by a lot of people and it makes a statement that you actually don't care that much about the discussion.